Process for the production of elemental fluorine by electrolysis

ABSTRACT

A process for the production of elemental fluorine by electrolysis of an electrolyte consisting essentially of hydrogen fluoride and an alkali metal fluoride in an electrolytic cell the cover of which is provided with gas separation skirts in which corrosion of the skirts is prevented by the addition to the electrolyte of a small proportion of an electrically conductive salt of barium or strontium that is capable of supplying barium or strontium ions to the said electrolyte.

United States Patent [191 Kaudewitz et al.

[4 1 Jan. 14, 1975 PROCESS FOR THE PRODUCTION OF ELEMENTAL FLUORINE BY ELECTROLYSIS [75] Inventors: Peter Kaudewitz; Hubert Friedrich,

both of Bad Wimpfen, Germany [73] Assignee: Kali-Chemie Fluor GmbH, Bad

Wimpfen,-Germany 22 Filed: Mar. 12, 1974 21 Appl. No.: 450,487

[30] Foreign Application Priority Data Mar. 13, 1973 Germany 2312297 [52] U.S. Cl. 204/59 R, 204/60 [51] Int. Cl 301k l/00 [58] Field of Search 204/59 R, 60

[56] References Cited UNITED STATES PATENTS 2,034,458 3/1936 Calcott et al7 204/59 R Howell et a1 204/60 Grubb 204/60 Primary ExaminerF. Edmundson Attorney, Agent, or Firm-Michael S. Striker [57] ABSTRACT 4 Claims, N0 Drawings PROCESS FOR THE PRODUCTION OF ELEMENTAL FLUORINE BY ELECTROLYSIS BACKGROUND OF THE INVENTION Elemental fluorine which has recently attained considerable industrial importance is produced commercially by electrolysis of anhydrous hydrofluoric acid (HF) which is also known as hydrogen fluoride. In order to increase the conductivity of the hydrogen fluoride and decrease its vapor pressure and inhibit polarization at the electrode and similarvundesirable effects that occur at the anode, potassium bifluoride or a similar electrically conductive salt is generally added to the electrolyte. However other electrically conductive salts such as fluorides of sodium, lithium, and ammonium are sometimes used for such purposes. The hydrogen fluoride and salt of which the electrolyte is composed must have high degrees of purity. They must be, for example, substantially free of water, compounds of sulfur, silicon, and heavy metals. As a general rule, however, an electrolyte containing a maximum of 0,1 percent by weight of water, for example, is considered to be acceptable.

Various electrolytic cells for generating fluorine are known which differ from each other generally only in structural details. The walls of the tanks of which such cells are constructed are generally formed of welded sheet steel or Monel Metal, or of steel sheets lined with Monel Metal. These walls generally serve as cathodes of the cell. The anodes are generally fastened to but electrically insulated from the lid or cell cover and the cell cover is generally cast and machined and is provided with skirts or partitions on its underside to separate the gas streams evolved at the anodes and cathodes from each other. These skirts or partitions are usually formed of nickel or nickel alloys such as Monel Metal or of a magnesium alloy, as they are enumerated in Kirk-Othmer-Encyclopedia of Chemical Technology,

2" Edition Vol. 12 pages 688 and 689 or in Ullmanns Encyklopadie der techn. Chemie, 3" Edition, Vol. 12

pages 109 and 111.

Corrosion of the cell cover is a serious problem affecting the operation of such electrolytic cells and occurs especially at the gas separation skirt as a result of which the material of which the skirts are formed dissolves in and consequently contaminates the electrolyte. When the corrosion of the skirt proceeds to such an extent that the skirt is pierced or perforated, the hydrogen in a cathode chamber can then mix with the fluorine in an adjacent anode chamber. An explosion can result from the reaction of the two gases in such a mixture. Frequent inspection of such skirts for evidences of corrosion is accordingly required to prevent the occurrences of explosions. For such inspections, the operation of the cell must be discontinued and the cell must be dismantled so that its cover can be removed and examined. In such explosions, carbon anodes are also generally ruined and the electrolyte is thereby contaminated with carbon particles and as a result cannot as such be reused. In a violent explosion such a rise in pressure may also be produced that the cover of the cell is blown off and the hot electrolyte is blown out of the cell, thereby causing serious injury to the operating personnel and considerable damage to the surrounding areas of the plant.

As reported by A. J. Rudge on page 55 of chapter 1 entitled Production of Elemental Fluorine by Electrolysis in the book Industrial Electrochemical Processes, edited by A. T. Kuhn and published in 1971 by Elsevier Publishing Co., Amsterdam, Netherlands, the normal life of a cell with a cover formed of a machined magnesium alloy casting is estimated, depending upon an assumed cell capacity of 2O 10 Ah, to be between 20 and 30 weeks.

SUMMARY OF THE INVENTION We have discovered that, by adding barium or strontium ions or mixture of both ions to the electrolyte in a fluorine-generator cell provided with a lid or cover with gas separator skirts on its underside the corrosion of the cells as well as the gas separating skirts thereon can be considerably inhibited as a result of which the lives of such cells is increased manyfold.

The process of the present invention for the production of elemental fluorine by electrolysis of a mixture of hydrogen fluoride and potassium bifluoride or similar fluoride salt in such electrolytic cells provided with gas separation skirts and covers accordingly comprises adding a barium or strontium salt or mixture thereof that is capable of supplying barium or strontium ions to the said electrolyte. It has been found that the rate of corrosion of the skirts can be reduced to from onetenth to one-hundredth of its original rate by the presence of the barium or strontium ions in the electrolyte, when the salt is present in an amount that is equivalent to at least 0.05 and at most 0.9 percent, and preferably at least 0.10 and at most 0.35 percent by weight of the electrolyte, these percentages pertaining to the equivalent content of barium or strontium ions or both ions in the salt. Preferably the barium or strontium ions are added as barium or strontium fluoride.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION The present invention is further described in connection with the following examples which are preferred embodiments thereof and which were selected solely for purposes of illustration and are accordingly not to be construed as limiting the invention.

. EXAMPLE 1 A new electrolytic fluorine generator cell of the type referred to as the Allied Chemical Corporation fluorine generator cell which is illustrated on page 513 of the Kirk-Othmer Encyclopedia of Chemical Technology Second Edition, Vol. 9, published in 1966 by Interscience Publishers, a division of John Wiley and Sons, Inc., New York, was used in this example.

The shell or walls of the cell were formed of steel plates. The lid or cell cover was a machined casting formed of a magnesium alloy containing more than percent by weight magnesium on the underside of which two gas separation skirts of the same material were arranged longitudinally and located appropriately with respect to the side of the shell and its longitudinal to produce elemental fluorine and hydrogen was continously replaced by fresh quantities.

The electrolyte that was maintained in this cell con sisted of potassium acid fluoride having the formula KF-2HF which contained barium ions in an amount equivalent to 0,15 percent by weight, which had been added as barium fluoride in hydrogen fluoride, which electrolyte had a hydrogen fluoride content between 40 and 42 percent by weight. The hydrogen fluoride had an average purity of 99.90 percent HF by weight and its average water content was less than 0.02 percent by weight.

The cell was operated in this manner for a period corresponding to 26 months, after which the magnesium content of the electrolyte was determined and found to be 0.008 percent by weight. Inspection of the gas separation skirts on the underside of the cell cover revealed that no substantial amount of corrosion of the magnesium alloy of which the cell cover and skirts was composed had occurred during this period.

EXAMPLE 2 A similar result as was described in Example 1 was observed when strontium fluoride instead of barium fluoride was added to the electrolyte in an amount corresponding to 0.12 percent by weight (calculated as strontium ion) of the total weight of the electrolyte. At the end of the 26-month operating period, the electrolyte contained a negligible proportion of magnesium and no substantial amount of corrosion of the cell cover and skirt was found.

COMPARATIVE EXAMPLE When another new cell that was identical with that referred to in Example 1 was operated in the same manner as described therein, but an electrolyte was used whichcontained no barium or strontium ions, the magnesium content of the electrolyte after 2 months of operation was 0.81% and the gas separation skirts were very heavily corroded.

Theforegoing examples, which show the differences in the extent of contamination of the electrolyte with magnesium that originated from the corrosion of the magnesium alloy of which the cell cover and skirts was composed, demonstrates the effectiveness of the addition of barium and strontium ions to the electrolyte to reduce considerably the corrosion of the magnesium alloy during operation of such a process in such an apparatus for the production of elemental fluorine.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features which, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and rangeof equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. A process for the production of elemental fluorine by electrolysis of an electrolyte consisting essentially of hydrogen fluoride and an alkali metal fluoride in which process the electrolyte contains barium or strontium ions or a mixture of both.

2. A process as defined in claim 1 in which the electrolyte contains the said barium or strontium ions in an amount between 0,05 and 0,9 percent by weight of the electrolyte.

3. A process as defined in claim 1 in which the electrolyte contains the said barium or strontium ions in an amount between 0,10 and 0,35 percent by weight of the electrolyte.

4. A process as defined in claim 1 in which the barium or strontium ions are present as a fluoride of barium or strontium. 

2. A process as defined in claim 1 in which the electrolyte contains the said barium or strontium ions in an amount between 0,05 and 0,9 percent by weight of the electrolyte.
 3. A process as defined in claim 1 in which the electrolyte contains the said barium or strontium ions in an amount between 0,10 and 0,35 percent by weight of the electrolyte.
 4. A process as defined in claim 1 in which the barium or strontium ions are present as a fluoride of barium or strontium. 